1. Field of the Invention
The invention relates to a power storage unit having a case, a plurality of power storage modules provided in the case, and an insulative liquid provided in the case. The invention relates also to a vehicle incorporating such a power storage unit.
2. Description of the Related Art
A typical battery pack is constituted of a plurality of battery cells (e.g., secondary batteries) electrically connected in series and disposed in a battery pack case. For example, a battery pack is known which has a battery pack case containing an insulative oil to facilitate the heat radiation from the battery cells and to insulate the battery cells from each other (Refer to Japanese Patent Application Publications No. 10-247513 (JP-A-10-247513), No. 2001-307691 (JP-A-2001-307691), No. 2001-60466 (JP-A-2001-60466), No. 2003-51298 (JP-A-2003-51298), and No. 09-17401 (JP-A-09-17401)).
Each battery cell is constituted of a battery case and a power generation element disposed in the battery case. In the event of an abnormality of the battery cell (e.g., when the battery cell is overcharged), gas may be produced by the power generation element in the battery cell. In view of this, there are battery cells having a valve for discharging the gas produced by the power generation element to the outside of the battery case. Note that the power generation element of each battery cell is constituted of a positive electrode plate, a negative electrode plate, and an electrolyte solution, and it is charged and discharged as needed.
In a battery pack having battery cells with the valves described above, in the event of an abnormality of any battery cell, gas is produced in said battery cell and then discharged therefrom. At this time, part of the power generation element (i.e., part of the electrolyte solution) in the battery cell may leak to the outside together with the gas. Although the battery pack case is filled with the insulative oil and therefore the respective battery cells are insulated from each other as mentioned above, if the electrolyte solution leaks to the outside of any battery cell, the insulation of the insulative oil weakens accordingly.
If the insulation of the insulative oil weakens, it may cause current to flow between the battery cell producing the gas and other battery cells, and this increases the temperature of each battery cell. An increase the temperature of each battery cell induces gas production therein, causing the valve at each battery cell to open.
As such, if the valve of any battery cell is opened, it may cause the valves of other battery cells to be opened. In this case, the pressure in the battery pack case increases excessively.
One option to solve this issue is to form the battery pack case to be strong enough to withstand such a pressure increase or to provide a gas guide mechanism for guiding the gas in the battery pack case to the outside.
However, increasing the strength of the battery pack case inevitably results in complication of the structure of the battery pack case, which leads to enlargement of the battery pack, or in a heavier weight of the battery pack case. Further, having a gas guide mechanism inevitably results in enlargement of the battery pack for accommodating said mechanism, which reduces the freedom in arranging the battery pack.
For example, such enlargement of the battery pack is not desirable especially in a case where the battery pack is mounted in a vehicle because it may result in enlargement of the vehicle, or the like.